As shown in FIG. 6, a vertical mill 1 equipped with a rotatable table 3A and grinding rollers 4 has found extensive use as a mill for pulverizing limestone, raw material of cement, and other raw material into powder. The rotatable table 3A takes the form of a disk and is rotated at a low speed by an electric motor 2A via a reduction gear 2 below a cylindrical casing 15. The grinding rollers 4 are circumferentially spaced from each other on the upper surface of the table 3A near the outer periphery of the table and hydraulically or otherwise pressed against the upper surface. The rollers 4, therefore, follow rotation of the table 3A.
Each grinding roller 4 is connected to the piston rod 10 of a hydraulic cylinder 9 via arms 5 and 7. The arm 5 is mounted to the casing 15 by a shaft 6 so as to be swingable. The cylinder 9 is operated to press the rollers 4 against the rotatable table 3A. As a result, pressure is applied to the raw material, for pulverizing it. A dam ring 3B is formed at the outer fringe of the table 3A to adjust the thickness of the layer of the raw material. An annular space 14 is formed around the table 3A to permit gas to be ejected upward. A rotating separator 13 classifies the particles formed by pulverizing the raw material by blades 13A. The finished product is taken out through a discharge port 16, together with gas. The raw material enters into the mill through a chute 15B.
FIG. 7 shows another conventional vertical mill which is similar to the mill shown in FIG. 6 except that the chute 15B for introducing a raw material is inserted in the side surface of the casing 15.
The vertical mill shown in FIG. 7 is particularly shown in FIG. 8. The mill has four grinding rollers 4A-4D mounted on the rotatable table 3A and regularly spaced from each other circumferentially. Hydraulic cylinders 9A-9D are connected with the grinding rollers 4A-4D, respectively. A hydraulic system 16 has a single hydraulic pressure-adjusting device which supplies hydraulic force to chambers at the ends of the cylinders 9A-9D via pressure lines 17. The same pressure is applied to all the rollers 4A-4D to crush the raw material. Accumulators 19 are connected with the pressure lines 17.
In this vertical mill, the raw material is supplied to the central portion of the rotatable table 3 through the chute 15B. Two kinds of force are applied to the raw material on the table 3A. One kind is a circumferential force produced by rotation of the table 3A. The other is a centrifugal force produced by circumferential movement of the raw material that is subjected to the circumferential force. The radial force and the circumferential force are combined to cause the raw material to move toward the outer periphery of the table 3A while drawing a vortical orbit on the table 3A. Since the raw material slips on the upper surface of the table 3A, the peripheral speed of the material is slightly less than the rotational speed of the table 3A.
Because the rollers are pressed against the upper surface of the table 3A near the outer periphery and rotated, the raw material drawing a vortical orbit enters between one roller 4 and the table 3A at a certain angle to the axial direction of the roller and are crushed. Then, the crushed material is forced under the following roller and ground. In this way, the diameters of particles of the raw material gradually decrease.
Gas such as air or hot air stream is introduced into the base portion of the casing 15 through a duct. The gas flows upwardly through the annular space 14 between the outer periphery of the table 3A and the inner surface of the casing. The material pulverized on the upper surface of the table 3A is about to drop into the annular space 14 from the fringe of the table 3A, but the powder is carried by the gas and rises inside the casing 15. The powdered material is classified by the blades 13A of the separator 13 located at a high position in the mill. Finished product of a given grain size is discharged from the discharge port 16 together with the gas and sent to the next station. Particles of greater grain sizes which dropped into the annular space 14 from the fringe of the table 3A are collected. Subsequently, the large particles are returned onto the table 3A through the chute 15B.
In the aforementioned vertical mill, the rotating table 3A cooperates with the grinding rollers 4 to crush the raw material. Where the raw material tends to slip and behaves like fluid, supply of the material to the rollers may become intermittent, thus greatly deteriorating the crushing efficiency. Especially, where the crushed material contains a small percentage of moisture such as calcium carbonate or slag, the particles of the material behave like fluid and tend to slip on the rotatable table. Therefore, each individual particle tends to separate from each other. For this reason, the raw material which is supplied to the central portion of the table from the chute is moved to one grinding roller and again pressed against the table at a given hydraulic pressure. Then, the material is rummaged by the rollers to which crushing force is given and so the material tends to escape laterally. As a result, the proportion of the material which is not forced between any one roller and the table increases. Also, a mass of the raw material is supplied ahead of each roller, because the passage of the material is stopped. Such a mass is moved under one roller at a time. In this way, it is highly unlikely that the raw material is stably and continuously supplied to the pulverizing rollers. After the raw material is sandwiched between one roller and the table and passes through this space, the material is suddenly released from the pressure. Therefore, the material pops up from the gap between the roller and the table. Consequently, the particles of the material become separate, thus forming an uncompacted layer. As such, the raw material may arrive at the next grinding roller without forming a stable layer. The result is that a sufficient amount of material is not supplied to the rollers. Hence, the pulverizing efficiency is very low.
In the vertical mill which pulverizes raw material between the rotating table and each grinding roller, the amount of material caught by each grinding roller and the size of the caught particles differ among the rollers, according to the condition of the particles of the material on the table. The loads on the rollers may differ. These differences are affected by the water content, the gain size, and other properties of the pulverized raw material. In this case, if the material is crushed by applying the same pressure to all the grinding rollers, overgrinding tends to occur, or it is difficult to fabricate products of a given grain size stably. Therefore, an excessive amount of power is consumed, or the mill produces vibration. This hinders stable operation of the mill. Especially, where slag or finished cement is crushed, the grain size distribution is required to be stable, because it is the final product.
The present applicant has already proposed a method of preventing overgrinding and reducing the amount of consumed power in Japanese Patent Laid-Open No. 147,649/1984. In this method, one set of two symmetrically arranged grinding rollers is controlled by a hydraulic controller. Specifically, their pulverizing pressure is controlled. Also, a second set of two symmetrically arranged grinding rollers is controlled by another hydraulic controller. The pulverizing pressures applied to two symmetrically arranged rollers of each set are the same and so this method cannot cope with the case where different loads are placed on the several rollers, for example four rollers. Hence, this method still cannot prevent overgrinding. Also, neither stabilization of the distribution of grain sizes of the finished product nor stable operation producing less vibration can be attained.